Method for controlling charging voltage of 12v auxiliary battery for hybrid vehicle

ABSTRACT

The present invention provides a method for controlling a charging voltage of a 12V auxiliary battery for a hybrid vehicle, which can (1) improve charging efficiency of an auxiliary battery by increasing the output voltage of a DC-DC converter during cold start when the outside air temperature is low, (2) improve the charging efficiency of the auxiliary battery by increasing or decreasing the output power of the DC-DC converter according to the state of charge of the auxiliary battery and by increasing the output voltage of the DC-DC converter when many electrical loads are turned on, and (3) allow the DC-DC converter to provide continuous power supply for charging the auxiliary battery by turning on a main switch disposed between a high voltage battery and the DC-DC converter based on a reverse power conversion operation of the DC-DC converter even in the case where the voltage of the auxiliary battery falls below 9V.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Applications Nos. 10-2009-0119494 filed Dec. 4, 2009 and10-2010-0072886 filed Jul. 28, 2010, the entire contents of which areincorporated herein by references.

BACKGROUND

(a) Technical Field

The present disclosure relates generally to a method for controlling acharging voltage of an auxiliary battery, particularly a 12V auxiliarybattery for a hybrid vehicle. More particularly, it relates to a methodwherein the power conversion of a DC-DC converter is controlled based onfactors such as the outside air temperature, the state of charge of theauxiliary battery, and the power consumption of electrical loads.

(b) Background Art

Hybrid vehicles are the vehicles of the future that employ an electricmotor as an auxiliary power source as well as a gasoline engine toprovide a reduction in exhaust gas and an improvement in fuelefficiency.

When the engine operates in an inefficient state, the electric motor isdriven by the power of a battery to increase the efficiency of a hybridsystem (load leveling). During deceleration, the battery is charged byregenerative braking, in which the kinetic energy, which would normallybe dissipated as frictional heat in a brake system, is converted intoelectrical energy by the power generation of the motor. As such, thefuel efficiency is improved.

Hybrid vehicles are divided into soft type hybrid vehicles and hard typehybrid vehicles based on whether or not the motor is connected anddriven in a power transmission system.

A motor drive system for an existing hard type hybrid vehicle is shownin FIG. 5. As shown in FIG. 5, the motor drive system includes first andsecond motors M1 and M2 for driving the vehicle; first and secondinverters 10 and 12 for driving the first and second motors M1 and M2,respectively; a DC high voltage battery 2 for applying a DC voltage fordriving the motors M1 and M2 to the first and second inverters 10 and12; a voltage converter 14 for boosting the DC voltage from the battery2 and supplying the boosted voltage to the first and second inverters 10and 12, and for lowering the DC voltage from the first and secondinverters 10 and 12 and supplying the lowered voltage to the battery 2;and a DC-DC converter 1 connected to the battery 2 for converting thevoltage of the DC power source.

FIG. 1 shows a system for charging a 12V auxiliary battery for a hybridvehicle and supplying power to electrical loads. As shown, a highvoltage battery 2 is connected to a DC-DC converter 1 through a mainswitch 3. An output terminal of the DC-DC converter 1 is connected to a12V auxiliary battery 8 and 12V electrical loads 4 (such as a variety ofcontrollers, headlights, a water pump, a radiator cooling fan, etc.),which are driven by the power of the 12V auxiliary battery 8. As furthershown, a junction box 6 is connected between the DC-DC converter 1 andthe electrical loads 4 and between the 12V auxiliary battery 8 and theelectrical loads 4. A wiring (parasitic) resistor 7 at the auxiliarybattery side is disposed between the DC-DC converter 1 and the 12Vauxiliary battery 8, and between the 12V auxiliary battery 8 and thejunction box 6. A wiring (parasitic) resistor 5 is also disposed betweenthe electrical loads 4 and the junction box 6.

In FIG. 1, V_(DC) represents the output voltage of the DC-DC converter1, V_(:J) represents the voltage of the junction box 6 applied to theelectrical loads 4 when the current of the electrical loads 4 is low,and V_(B) represents the charging voltage of the 12V auxiliary battery8. The magnitude of the voltage is in the order of V_(DC)>V_(:J)>>V_(B).

The start-up sequence of a hybrid vehicle typically includes a step inwhich the ignition switch (IG) is turned on by a driver, a step in whichthe various controllers and the main switch 3 are turned on by powerfrom the 12V auxiliary battery 8 (the DC-DC converter 1 is not operateduntil the main switch 3 is turned on), and a step in which the DC-DCconverter 1 operates upon completion of the start-up to charge theauxiliary battery 8 and to supply electrical power to the electricalloads 4.

In a hybrid vehicle (such as a fuel cell vehicle, a plug-in hybridvehicle, and an electric vehicle) having the above configuration andoperation, if the voltage of the DC-DC converter is maintained at a lowlevel, the power consumption of the electrical loads is reduced, whichimproves the fuel efficiency. However, the amount of electrical energycharged in the auxiliary battery is reduced, which reduces the chargingefficiency, thereby causing a failure during cold start-up. Inparticular, the DC-DC converter in the hybrid vehicle performs thefunctions of charging the 12V auxiliary battery and supplying power tothe electrical loads of the vehicle. To improve the fuel efficiency, itis necessary to maintain the voltage supplied to the electrical loads ata lower level. It is also advantageous to increase the charging voltagewithin an allowable range to improve the charging efficiency of theauxiliary battery. However, if the auxiliary battery charge is low, thevehicle may not start during cold start-up due to the reduction inbattery voltage, which may reduce the quality of the vehicle. Inparticular, as shown in FIG. 1, if the distance between the DC-DCconverter 1 and the electrical loads 4 is short, and if the distancebetween the DC-DC converter 1 and the auxiliary battery 8 is long, thenthe output voltage range of the DC-DC converter 1 is increased, whichfurther reduces the power conversion efficiency.

While the diameter of the power cable between the DC-DC converter andthe auxiliary battery may be increased in an attempt to address thisproblem, this results in an increased weight of the vehicle, increasedmanufacturing costs, and a reduction in the degree of freedom in cablelayout. Alternatively, while the DC-DC converter and the auxiliarybattery may be disposed adjacent to each other and, at the same time,the electrical loads may be spaced away from each other in an attempt toaddress this problem, this also reduces the degree of freedom in vehicledesign and, further, reduces the quality of the vehicle due to variouslimitations such as cooling of the DC-DC converter and the like.

As mentioned above, the DC-DC converter 1 operates upon completion ofthe start-up to charge the auxiliary battery 8 and to supply electricalpower to the electrical loads 4. At this time, if the voltage of theauxiliary battery is less than 9V, the main switch 3 generally may notbe turned on (however, it is noted that various controllers normallyoperate at a voltage of more than 6V), and thus the DC-DC converter isnot connected to the high voltage battery 2. As a result, the auxiliarybattery is not charged, and the voltage of the auxiliary battery may befurther reduced during the cold start-up.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

The present invention provides a method for controlling a chargingvoltage of an auxiliary battery for a hybrid vehicle. In particular, thepresent invention improves the charging efficiency of an auxiliarybattery by increasing the output voltage of a DC-DC converter,particularly during cold start when the outside air temperature is low.It is noted that while the present methods are described with particularreference to 12V auxiliary batteries, the methods could be suitablymodified and applied to different voltage batteries.

In accordance with one aspect of the present invention the output powerof a DC-DC converter is increased or decreased according to the state ofcharge of the auxiliary battery, and the output voltage of the DC-DCconverter is increased when many electrical loads are turned on. Assuch, charging efficiency of the auxiliary battery is improved.

In another aspect of the present invention, the DC-DC converter isconfigured to provide a continuous power supply for charging theauxiliary battery. In particular, a main switch is disposed between ahigh voltage battery and the DC-DC converter, and the switch is turnedon based on a reverse power conversion operation of the DC-DC convertereven when the voltage of the auxiliary battery falls below a lowerlimit, such as 9V for the case of a 12V auxiliary battery.

In an exemplary embodiment, the present invention provides a method forcontrolling a charging voltage of a 12V auxiliary battery for a hybridvehicle, the method including: comparing the outside air temperaturewith a boundary temperature after the vehicle is started; if the outsideair temperature is lower than the boundary temperature, measuring thevoltage of the 12V auxiliary battery; and increasing the output voltageat a DC-DC converter if the voltage of the 12V auxiliary battery isbeyond a predetermined reference voltage range.

In another aspect, the present invention provides a method forcontrolling a charging voltage of a 12V auxiliary battery for a hybridvehicle, the method including: comparing the current amount of 12Velectrical loads with a reference load value after the vehicle isstarted; if the amount of 12V electrical loads is smaller than thereference load value, increasing the output voltage of the DC-DCconverter to improve charging efficiency of a 12V auxiliary battery; ifthe amount of 12V electrical loads is greater than the reference loadvalue, determining the state of charge of the 12V auxiliary battery; andif the state of charge of the 12V auxiliary battery is lower than acharge boundary value, enhancing the charging speed of the 12V auxiliarybattery.

In still another aspect, the present invention provides a method forcontrolling a charging voltage of a 12V auxiliary battery for a hybridvehicle, the method including: comparing the voltage of the 12Vauxiliary battery with a low limit value for operation of a main switch(which connects a high voltage battery to a DC-DC converter) and avariety of controllers after the vehicle is started; if the voltage ofthe 12V auxiliary battery falls below the low limit value, allowing theDC-DC converter to perform a reverse power conversion; allowingelectrical energy to be transferred from the 12V auxiliary battery to ahigh voltage capacitor for storage; allowing the DC-DC converter toperform a forward power conversion for a predetermined short period oftime; and allowing high voltage energy of the high voltage capacitor tobe supplied to the 12V auxiliary battery through the DC-DC converter.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The above and other features of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a schematic diagram of a system for charging a 12V auxiliarybattery for a hybrid vehicle and supplying power to electrical loads.

FIG. 2 is a flowchart illustrating a method for controlling a chargingvoltage of a 12V auxiliary battery for a hybrid vehicle in accordancewith an exemplary embodiment of the present invention.

FIG. 3 is a flowchart illustrating a method for controlling a chargingvoltage of a 12V auxiliary battery for a hybrid vehicle in accordancewith another exemplary embodiment of the present invention.

FIG. 4 is a flowchart illustrating a method for controlling a chargingvoltage of a 12V auxiliary battery for a hybrid vehicle in accordancewith still another exemplary embodiment of the present invention.

FIG. 5 is a schematic diagram of a motor drive system for a hybridvehicle.

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

1: DC-DC converter 2: high voltage battery 3: main switch 4. electricalloads 6: junction box 5 & 7: parasitic resistors 8: 12 V auxiliarybattery 10 & 12: inverters 14: voltage converter 16: DC-link capacitor

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

As mentioned above with reference to FIG. 1, in a system for charging a12V auxiliary battery and supplying power to the electrical loads 4, thehigh voltage battery 2 is connected to the DC-DC converter 1 through themain switch 3, and the output terminal of the DC-DC converter 1 isconnected to the 12V auxiliary battery 8 and the 12V electrical loads 4.Also, as shown in FIG. 5, in the motor drive system for the hybridvehicle, the first and second inverters 10 and 12 for driving the motorsM1 and M2 are connected to an input terminal of the DC-DC converter 1through the voltage converter 14 and a DC-link capacitor 16.

The present invention provides methods for controlling a chargingvoltage of a 12V auxiliary battery for a hybrid vehicle by controllingthe power conversion of the DC-DC converter based on the outside airtemperature, the state of charge of the auxiliary battery, and the powerconsumption of electrical loads.

An exemplary embodiment of the present invention will be described withreference to FIG. 2 which shows a method for controlling the chargingvoltage of the 12V auxiliary battery for the hybrid vehicle. This methodprevents the reduction of the auxiliary battery voltage at lowtemperatures, and, thus, the reduction in charging efficiency duringcold start. If the ambient temperature of the vehicle is low (forexample, below 0° C.) during operation of the DC-DC converter, thepresent method increases the output voltage of the DC-DC converter and,at the same time, increase the voltage supplied from the DC-DC converterto the auxiliary battery, regardless of the reduction in fuelefficiency. As a result, the charging speed of the auxiliary battery isenhanced.

In a preferred embodiment, after the vehicle is started, it isdetermined whether the outside air temperature Tx is lower than aboundary temperature Tx1. If the outside air temperature is higher thanthe boundary temperature, the DC-DC converter is controlled to operatein a normal mode.

On the other hand, if it is determined that the outside air temperatureis lower than the boundary temperature (for example, below 0° C.), thevoltage at the output terminal of the DC-DC converter or the voltage Vbof the 12V auxiliary battery is measured. If the voltage at the outputterminal of the DC-DC converter or the voltage Vb of the 12V auxiliarybattery is lower than a predetermined reference voltage V2, the DC-DCconverter is controlled to operate in a rapid charge mode tosignificantly increase its output voltage. If the voltage at the outputterminal of the DC-DC converter or the voltage Vb of the 12V auxiliarybattery is higher than the predetermined reference voltage V2, the DC-DCconverter is controlled to operate in a trickle charge mode to slightlyincrease its output voltage.

As such, if the outside air temperature falls below the boundarytemperature, the present method increases the output voltage of theDC-DC converter to facilitate the charging of the 12V auxiliary battery.Thus, in the case where the state of charge of the 12V auxiliary batteryis low, it is possible to prevent a situation in which the vehicle doesnot start during cold start-up due to the reduction in the voltage ofthe auxiliary battery.

Subsequently, the current charged into or discharged from the 12Vauxiliary battery is measured. If the accumulated absolute value IB_SUMof the charge or discharge current is higher than a boundary value I1,the DC-DC converter is controlled to operate in a normal mode. If it islower than the boundary value I1, the DC-DC converter is controlled tooperate in a mode in which its output voltage is significantly orslightly increased.

Another exemplary embodiment of the present invention will be describedwith reference to FIG. 3 which shows a method for controlling thecharging voltage of the 12V auxiliary battery for a hybrid vehicle. Inthis embodiment, the charging efficiency of the auxiliary battery isimproved by increasing or decreasing the output voltage of the DC-DCconverter according to the state of charge of the auxiliary battery,because the voltage of the auxiliary battery is determined according tothe state of charge of the auxiliary battery. In particular, if thestate of charge of the auxiliary battery is high, then the outputvoltage of the DC-DC converter is decreased to improve the fuelefficiency. On the other hand, if the state of charge of the auxiliarybattery is low, the output voltage of the DC-DC converter is increasedregardless of the reduction in fuel efficiency, thereby enhancing thecharging speed of the auxiliary battery.

In accordance with this embodiment, when the amount of electrical loadsbeing operated is low, the DC-DC converter is controlled to operate in amode for enhancing the charging speed of the auxiliary battery In thiscase, an increase in the power consumption of the electrical loads issmall even though the output voltage of the DC-DC converter isincreased. On the other hand, when the amount of electrical loads beingoperated is high, the DC-DC converter is controlled to operate in a modefor improving the fuel efficiency in which the output voltage of theDC-DC converter is decreased.

In a preferred embodiment, after the vehicle is started, the amount of12V electrical loads being operated is determined, and the amount of 12Velectrical loads being operated is compared with a reference load valueL1.

If the amount of 12V electrical loads being operated is lower than thereference load value L1, the output voltage of the DC-DC converter isincreased to facilitate the charging of the 12V auxiliary battery. Inother words, a mode for enhancing the charging speed of the auxiliarybattery is performed. In this case, where the amount of 12V electricalloads being operated is lower than the reference load value L1, theincrease in the amount of electrical loads is relatively small, eventhough the output voltage of the DC-DC converter is increased, and theamount of electrical loads is proportional to the voltage. Therefore,the charge of the 12V auxiliary battery is advantageously made.

On the other hand, if the amount of 12V electrical loads being operatedis higher than the reference load value L1, the state of charge of the12V auxiliary battery SOC_A is determined. If the state of charge of the12V auxiliary battery SOC_A is lower than a charge boundary value S1,the method proceeds to a mode for enhancing the charging speed of theauxiliary battery. On the other hand, if the state of charge of the 12Vauxiliary battery SOC_A is higher than the charge boundary value S1, themethod proceeds to a mode for improving the fuel efficiency, in whichthe output voltage of the DC-DC converter is decreased. In particular,if the amount of 12V electrical loads is higher than the reference loadvalue L1, it means that a large number of electrical loads are operated.Thus, it is possible to reduce the power consumption of the electricalloads by decreasing the output voltage of the DC-DC converter.

Another exemplary embodiment of the present invention will be describedwith reference to FIG. 4 which shows a method for controlling thecharging voltage of the 12V auxiliary battery for a hybrid vehicle. Inthis embodiment, as shown and described in connection with FIG. 1, ifthe voltage of the 12V auxiliary battery falls below 9V, the main switch3 is not turned on, and thus the DC-DC converter 1 does not use thepower of the high voltage battery 2. Therefore, in this embodiment, theauxiliary battery is charged by applying a momentary high voltage to theauxiliary battery such that the auxiliary battery can apply a voltage ofmore than 9V to the main switch. In particular, the DC-DC converterperforms a reverse power conversion from the auxiliary battery to thehigh voltage capacitor (i.e., the DC-link capacitor connected betweenthe voltage converter, which is connected to the high voltage battery,and the inverter) such that the electrical energy of the auxiliarybattery is charged into the high voltage capacitor. After sufficientadditional electrical energy is stored in the high voltage capacitor,the DC-DC converter performs a forward power conversion from the highvoltage capacitor to the auxiliary battery for a short period of timesuch that a high voltage (e.g., about 14V) is supplied from the highvoltage capacitor to the auxiliary battery. As a result, the auxiliarybattery applies a voltage of more than 9V to the main switch.

In an exemplary embodiment, after the vehicle is started, the voltage Vbof the 12V auxiliary battery is compared with a low limit value V1 forthe operation of the main switch (i.e., relay), which connects the highvoltage battery to the DC-DC converter, and a variety of controllers. Ifthe voltage Vb of the 12V auxiliary battery falls below the low limitvalue V1 (for example, if the voltage of the 12V auxiliary battery fallsbelow 9V), the reverse power conversion operation of the DC-DC converteris performed. Based on the reverse power conversion operation of theDC-DC converter, the electrical energy is transferred from the 12Vauxiliary battery to the high voltage capacitor (for example, theDC-link capacitor or the capacitor at the input terminal of the DC-DCconverter). Thus, much more electrical energy is further stored in thehigh voltage capacitor. Subsequently, the DC-DC converter performs aforward power conversion for a predetermined short period of time suchthat a high voltage (e.g., about 14V) of the high voltage capacitor issupplied to the 12V auxiliary battery through the DC-DC converter andcharged thereinto.

As such, according to the principle that when the electrical energy ischarged in the auxiliary battery in the reverse direction for arelatively long period of time and is then transferred in the forwarddirection for a short period of time, the voltage is increased. Amomentary normal charge of the 12V auxiliary battery is performed and,as a result, the 12V auxiliary battery applies a voltage of more than 9Vto the various controllers and the main switch, thereby turning on themain switch. Once the mail switch is turned on, the DC-DC converter isnormally driven by the power of the high voltage battery, thus supplyinga stable voltage for charging the 12V auxiliary battery.

The present invention, thus, provides methods for improving the chargingefficiency of an auxiliary battery and improving the cold startperformance even when outside air temperature is low by increasing theoutput voltage of the DC-DC converter. Further, if the state of chargeof the auxiliary battery is high, the fuel efficiency is improved bydecreasing the output voltage of the DC-DC converter. On the other hand,if the state of charge of the auxiliary battery is low, the chargingefficiency of the auxiliary battery is improved by increasing the outputvoltage of the DC-DC converter. Further, if the amount of electricalloads being operated is small, the charging efficiency of the auxiliarybattery is improved by increasing the output power of the DC-DCconverter. On the other hand, if the amount of electrical loads beingoperated is large, the fuel efficiency is improved by decreasing theoutput voltage of the DC-DC converter. Still further, even in the casewhere the voltage of the auxiliary battery falls below 9V, according tothe present invention, the main switch disposed between the high voltagebattery and the DC-DC converter is turned on based on the reverse powerconversion operation of the DC-DC converter such that the DC-DCconverter operates normally to thereby continuously charge the auxiliarybattery.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

1. A method for controlling a charging voltage of a 12V auxiliarybattery for a hybrid vehicle, the method comprising: comparing theoutside air temperature with a boundary temperature after the vehicle isstarted; if the outside air temperature is lower than the boundarytemperature, measuring the voltage of a 12V auxiliary battery; and ifthe voltage of the 12V auxiliary battery is beyond a predeterminedreference voltage range, increasing the output voltage at a DC-DCconverter.
 2. The method of claim 1, wherein if the voltage of the 12Vauxiliary battery is lower than the predetermined reference voltage, theDC-DC converter operates in a rapid charge mode in which the outputvoltage is significantly increased.
 3. The method of claim 1, wherein ifthe voltage of the 12V auxiliary battery is higher than thepredetermined reference voltage, the DC-DC converter operates in atrickle charge mode in which the output voltage is slightly increased.4. The method of claim 1, wherein if the outside air temperature ishigher than the boundary temperature, the DC-DC converter operates in anormal mode.
 5. The method of claim 1, further comprising: measuring thecurrent charged into or discharged from the 12V auxiliary battery; ifthe accumulated absolute value of the charge or discharge current ishigher than a boundary value, controlling the DC-DC converter to operatein a normal mode; and if the accumulated absolute value of the charge ordischarge current is lower than the boundary value, controlling theDC-DC converter to operate in a mode in which its output voltage issignificantly or slightly increased.
 6. A method for controlling acharging voltage of a 12V auxiliary battery for a hybrid vehicle, themethod comprising: comparing the current amount of 12V electrical loadswith a reference load value after the vehicle is started; if the amountof 12V electrical loads is smaller than the reference load value,increasing the output voltage of a DC-DC converter to perform a mode forimproving charging efficiency of a 12V auxiliary battery; if the amountof 12V electrical loads is greater than the reference load value,determining the state of charge of the 12V auxiliary battery; and if thestate of charge of the 12V auxiliary battery is lower than a chargeboundary value, entering a mode for enhancing the charging speed of the12V auxiliary battery.
 7. The method of claim 6, further comprising ifthe state of charge of the 12V auxiliary battery is higher than a chargeboundary value, entering a mode for improving fuel efficiency in whichthe output voltage of the DC-DC converter is saved by decreasing theoutput voltage of the DC-DC converter.
 8. A method for controlling acharging voltage of a 12V auxiliary battery for a hybrid vehicle, themethod comprising: comparing the voltage of a 12V auxiliary battery witha low limit value for operation of a main switch, wherein the mainswitch connects a high voltage battery to a DC-DC converter, and avariety of controllers after the vehicle is started; if the voltage ofthe 12V auxiliary battery falls below the low limit value, allowing theDC-DC converter to perform a reverse power conversion; allowingelectrical energy to be transferred from the 12V auxiliary battery to ahigh voltage capacitor and stored in the high voltage capacitor;allowing the DC-DC converter to perform a forward power conversion for apredetermined short period of time; and allowing high voltage energy ofthe high voltage capacitor to be supplied to the 12V auxiliary batterythrough the DC-DC converter.
 9. The method of claim 8, furthercomprising: allowing a voltage of more than a low limit voltage to beapplied from the 12V auxiliary battery to the main switch and thecontrollers to turn on the main switch; and allowing the DC-DC converterto be normally driven by the power of the high voltage battery due tothe turning on of the main switch.